1. Field of the Invention
This invention relates to digital signal processing apparatus for frequency demultiplexing or multiplexing.
2. Description of the Prior Art
The invention especially relates to such processing apparatus for use on-board a satellite. Typically, such a satellite will have receiving beams and transmitting beams. The on-board processor may demultiplex the received uplink channel them to different downlink channels where desired, multiplex the sub-bands of the respective downlink channels, change the frequency of the downlink channels, amplify the channels, that the downlink beams may be transmitted.
In the case of wideband channels, which are typically hundreds of Megahertz (MHz) in bandwidth, there are standard widths of sub-band into which the channel can be divided, for example, 36, 72, 108 MHz. Current analogue multiplexers or demultiplexers accordingly have filters appropriate to these bandwidths which are switched in remotely from the ground as required.
A problem with such an analogue implementation is the weight and volume occupied by the filters and switches, and digital schemes have been considered to overcome this.
The Applicants have previously proposed a digital signal processing apparatus for multiplexing or demultiplexing a narrowband channel, typically 4 MHz, with sub-bands typically 36 KHz in width (EP-A-0 695 054).
FIG. 2 shows the general scheme of the demultiplexer. The sub-bands are isolated by means of a polyphase filter and linked FFT units 4a, 4b, which perform a relatively coarse filtering operation.
The frequency response of the polyphase filter/FFT for extracting sub-band 0 is shown in FIG. 3b. The pass band is centered on the desired channel, but the transition bands are relatively relaxed and actually extend over the adjacent sub-band on each side. The units 4a, 4b have a similar frequency response centered on each sub-band K-1, 0, 1 etc of the narrowband input channel, which is a complex FDM (frequency division multiplex) of K-channels (FIG. 3a).
The relaxed transition bands reduce the complexity of the polyphase filter/FFT implementation, and hence reduce its cost.
Multiplexers/demultiplexers proposed before EP-A-0 695 054 overcame the problem of the relaxed transition bands by using a bank of fine (i.e. tight) digital filters on each sub-band extracted by the units 4a, 4b. However, this still left the problem that the relaxed transition bands restricted the amount by which each sub-band could be decimated in the polyphase filter/FFT. The maximum possible decimation was by K/2 (K being the number of sub-bands), as shown in FIG. 3c. Further decimation would alias the transition bands, which contain signal energy from adjacent channels, into the passband.
In EP-A-0 695 054, the Applicants proposed replacing the bank of fine filters by a block fine filtering stage before the coarse filtering stage. The first digital filter 3 consisted of an imaged low pass (for example half band) digital filter (producing an output as shown in FIG. 3e) and an imaged high pass (for example, half band) digital filter (producing an output as shown in FIG. 3f). This was obtained by using a digital filter such as an FIR filter having the desired tight (prototype) filter shape, say, that centered on sub-band k=0, and then padding the impulse response with zero coefficients to produce multiple images of that tight filter shape. The alternate frequency slots for the low pass case are nulled out (FIG. 3e). So are those for the high pass case (FIG. 3f). Each low and high pass prototype impulse response are each padded with (K/2-1) zeros between each coefficient to produce K/2 images of the prototype filter each.
When the block fine low and high pass filtering stage 3 precedes the block coarse processing stage (in fact stages, since a polyphase filter/FFT unit is provided for the output of each of the block low and high pass filters), the transition bands (FIG. 3b) now lie in the nulled out regions of the spectrum. Not only is the bank of fine filters unnecessary, but now the maximum possible decimation of K can be performed in each of the two polyphase filter/FFT units 4a, 4b, as shown in FIG. 3d. The transition bands of adjacent sub-bands are now wholly aliased with each sub-band, but this does not matter since they do not contain any energy.